Characterization of a Total Carbon Measurement for Use in Laboratory Chamber Experiments

Abstract:

The oxidative processing of reactive organic carbon (ROC) involves a very large number of unique species. This chemical complexity has precluded a robust understanding of the fates of organic emissions and remained a barrier to accurate modeling of secondary organic aerosol. In recent decades, progressively larger ranges of the total chemical domain of ROC have been successfully detected, owing to advancements in inlet design and the proliferation of ionization schemes for mass spectrometry. These measurement advances have abetted the utility of laboratory chamber experiments as an avenue towards understanding the ensemble behavior of processed ROC. However, in spite of this progress within the laboratory towards carbon closure—a complete accounting of the transformation of all precursor carbon—the quantification of loss processes to the walls of the chamber remains incomplete. In order to measure total suspended carbon in chamber experiments, we adapt a “total carbon” technique originally developed for the calibration of chemical ionization mass spectrometers. This measurement is performed by combining high temperature platinum and palladium catalysts to fully oxidize ROC to CO₂, followed by a differential CO₂ measurement to quantify the total carbon content. Here we characterize the fidelity of this setup (the “OxyCat”) in detecting a wide range of gaseous and particle-phase carbon-containing species over a range of relevant concentrations. Furthermore, we demonstrate the incorporation of the OxyCat in sampling directly from a laboratory chamber with a variable CO₂ background. The addition of this top-down carbon content observation to the suite of analytical tools available in the laboratory provides a valuable constraint towards the quantification of wall loss and the assessment of the remaining gaps and overlaps between different ROC mass spectrometric measurements.